This invention relates to the heat treatment of a titanium-alloy article and, more particularly, to the annealing heat treatment of the titanium-alloy article that forms a martensitic structure during prior processing steps.
The fabrication of a metallic article which has a range of section thicknesses and is made of an alloy whose properties depend upon cooling rate presents a manufacturing challenge. The thinner portions of the article cool faster than the thicker portions, so that the thinner portions have one set of properties and the thicker portions have another set of properties. It some cases it may be possible to use compensating cooling rates for the various portions or very slow cooling rates, but this adds considerable expense and is not always practical.
An example is the manufacture of a forged compressor blade for a gas turbine engine. The compressor blades may be made of a titanium alpha-beta alloy such as Ti-442, having a nominal composition, in weight percent, of about 4 percent aluminum, about 4 percent molybdenum, about 2 percent tin, about 0.5 percent silicon, balance titanium. This alloy forms a martensitic structure upon cooling, and the nature and extent of the martensite transformation depend upon the cooling rate. The material is heated to about 1650xc2x0 F., transferred to the forging dies, and forged at the starting temperature of about 1650xc2x0 F. The article cools in contact with the cooler forging dies. The thin airfoil portions of the compressor blade, and particularly the leading and trailing edges, cool rapidly and develop extensive martensite, while the thick dovetail portions cool more slowly and form little if any martensite. The martensite in the airfoil portion is relatively brittle and susceptible to impact damage and premature failure. Similar problems arise during the weld repair of articles made of these alloys that have been in service.
To overcome these problems and provide the desired combination of properties, various heat treatments have been developed and employed. In one, the hot-forged article is heat treated at 1650xc2x0 F. for one hour and slow cooled, followed by a low-temperature aging at 932xc2x0 F. for 24 hours. In another heat treatment, the hot-forged article is heat treated at 1020xc2x0 F. for 4 hours. Neither of these heat treatments has proved successful in imparting the required combination of a high-strength, fatigue-resistant dovetail and a more-ductile, damage-resistant finished airfoil that does not distort during processing.
Accordingly, there is a need for a heat treatment for hot-forged Ti-442 articles, and, more generally, for articles made of titanium-base alloys that form martensite or other cooling-rate-related microstructures upon cooling. The present invention fulfills this need, and further provides related advantages.
The present invention provides a heat treatment technique that is useful for heat treating alpha-beta titanium-base alloys, such as those with a relatively high molybdenum content, that form a martensitic structure upon rapid cooling. It is particularly useful in conjunction with Ti-442 alloy. The heat treatment procedure produces high strength and fatigue resistance in the thicker portions of the article (e.g., the dovetail in the preferred compressor blade application), and improved ductility, damage tolerance, fracture toughness, and ballistic-impact resistance in the thinner portions of the article (e.g., the airfoil and particularly the leading and trailing edges of the compressor blade). The thinner portions do not substantially distort during the heat treatment, so that rework of the article is minimized or avoided.
A method for heat treating an article comprises the steps of providing an article formed of a alpha-beta titanium-base alloy, and processing the article to form a martensitic structure therein. The step of processing includes the steps of first heating the article to a first-heating temperature of greater than about 1600xc2x0 F., and thereafter first cooling the article to a temperature of less than about 800xc2x0 F. The method further includes thereafter second heating the article to a second-heating temperature of from about 1275xc2x0 F. to about 1375xc2x0 F. for a time of from about 1 to about 7 hours (most preferably from about 4 to about 6 hours), and thereafter second cooling the article to a temperature of less than about 800xc2x0 F. at a second cooling rate that does not exceed about 15xc2x0 F. per second (and is usually from about 1xc2x0 F. per second to about 15xc2x0 F. per second). The second heating to the second-heating temperature is preferably to a temperature of about 1350xc2x0 F. for a time of about 6 hours. The second cooling is optionally followed by a step of stress relieving the article at a temperature of from about 1000xc2x0 F. to about 1050xc2x0 F., most preferably 1020xc2x0 F.+/xe2x88x9220xc2x0 F. for two hours.
The titanium-base alloy typically contains molybdenum in an amount exceeding about 3.5 percent by weight. In a preferred application, the titanium-base alloy is Ti-442 which has a nominal composition, in weight percent, of about 4 percent aluminum, about 4 percent molybdenum, about 2 percent tin, about 0.5 percent silicon, balance titanium. The total of all of the elements, including impurities and minor elements, is 100 percent by weight.
The present approach is most advantageously applied for articles that have thin portions and thick portions. For example, the article may have have a first portion with a thickness of less than about 0.2 inch and a second portion with a thickness of greater than about 0.2 inch. A gas turbine compressor blade is such an article, having a thin airfoil portion and a thick dovetail portion.
The processing that produces the martensitic structure involves heating to the first-heating temperature of greater than about 1600xc2x0 F. The processing may be a simple heat treatment, but it usually involves other operations as well. For example, in a new compressor blade the step of processing may include forging the article at the first-heating temperature, such as forging at a starting temperature of about 1650xc2x0 F. In a compressor blade that has previously seen service and has experienced removal of the blade tip or other damage to the airfoil portion, the step of processing may include weld repairing the article at the first-heating temperature, which is well in excess of 1600xc2x0 F. and up to the melting point of the alloy.
This family of alloys has not had a generally accepted annealing procedure in the past, and it was not recommended for use in the annealed condition. The present approach is based upon a recognition that the prior heat treatments used for these alloys have been developed primarily from experience with relatively thick pieces of material that do not have thin portions and thick portions. The prior approaches did not produce the desired combination of properties in the article with thin portions and thick portions. The prior heat treatment at 1650xc2x0 F. for one hour and slow cool, followed by a low-temperature aging at 932xc2x0 F. for 24 hours produced high distortion of the thin portions. The prior heat treatment at 1020xc2x0 F. for 4 hours produced the article with minimal distortion of the thin portion and a high-strength, fatigue-resistant dovetail, but the airfoil had too high a strength and insufficient damage tolerance and ballistic impact resistance. The present approach including the second heating, which serves as an annealing treatment, imparts improved properties to the finished article. Good damage tolerance and ballistic impact resistance is a necessary property of the compressor blade airfoils, because of the possibility of ingestion of foreign objects into the front end and compressor stages of the engine.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.